In general, inkjet printers include at least one printhead having a plurality of inkjets that eject drops of liquid ink onto a recording or image forming surface. In some inkjet printers, the printhead ejects ink directly onto the surface of media as the media passes the printhead. The media can be in the form of a continuous web or in the form of sheets. In continuous web printers, the media is pulled from a supply roll by actuator-driven rollers. As the web moves through the printer it passes around rollers to which tension is applied to keep the web taut as it passes through the printer to a take-up roll. In sheet printers, actuator-driven rollers are positioned against one another to form nips and these nips pull sheets from a media supply and propel them through the printer to an output tray.
In inkjet printers that eject ink directly onto sheets, media deformation occurs more frequently in sheet printers than continuous web printers since a web is generally taut as it passes through the printer. Sheets, however, can absorb moisture from the inks ejected onto the sheets and this moisture can cause curling or other deformations in the media sheets. These deformations are particularly troublesome in inkjet printers that employ water-based or solvent-based inks in which pigments or other colorants are suspended or are in solution with water or another solvent. The water and solvents in the inks can change the physical properties of the sheets in ways that degrade the quality of the images produced on the media sheets. In these aqueous ink printers, an unacceptable level of curl can be induced on the printed sheet by the image, particularly when a solid stripe of ink is printed on the leading edge of a sheet. To address the curl in a sheet, a device known as a decurler is used to induce curl in the sheet in the opposite direction to counteract the curl induced by the printed image on the sheet. This function is important, particularly when the printed sheet is delivered to an in-line stacker, which can only handle sheets having curl no greater than a predetermined radius.
A decurler in an aqueous inkjet printer has an indent roller and an elastomeric roller. An actuator operatively connected to the elastomeric roller pushes the roller into the indent roller as the rotation of the two rollers passes a sheet between them. This action induces curl into the sheet in the opposite direction of the curl induced by the ink image on the sheet to reduce the curl in the sheet to a level that enables the stacker to handle the sheet. Because the indent roller contacts the ink on the freshly printed side of the media, some of the ink can adhere to the roller. The ink adhering to the indent shaft can build to levels that adversely impact the ability of the decurler to induce opposite curl in the media sheets and may wrinkle the sheet in the decurler. Generally, wrinkled sheets are not acceptable to printer users. The appearance of wrinkled sheets in the output tray requires printer down time for maintenance of the decurler to remove the ink from the indent roller and the discarding of the wrinkled sheets. Detection of ink adherence to the indent roller would enable maintenance to be performed on the indent roller before the printed sheets begin to wrinkle and be discarded.